The present invention relates to a piezoelectric material comprising poly(D-lactic acid)/poly(L-lactic acid) stereocomplex crystals and, more particularly, to a piezoelectric material, which is configured such that poly(D-lactic acid) and poly(L-lactic acid) are crystallized into a stereocomplex to thus exhibit superior heat resistance and piezoelectric properties.
With the rapid spread of tablet personal computers and smart phones, the development of flexible wearable devices is required. For wearable devices, polymer-based soft sensors and actuators are being developed. A piezoelectric polymer and an electrospun nanofiber web, such as polyvinylidene fluoride (PVDF) and its copolymer (PVDF-TrFE)-based film, have been utilized in a variety of fields, including those of biomedical sensors, filters, energy production, etc. PVDF is known to exhibit superior piezoelectric properties, and has to possess a larger number of oriented C—F dipoles and to have a higher net dipole moment per unit area while increasing β-crystallization, in order to effectively serve as piezoelectric sensors, generators, and actuators.
Although PVDF advantageously manifests piezoelectric properties, PVDF-based wearable sensors are problematic due to the pyroelectric properties thereof. When they are touched by the hands or other body parts, it is impossible to distinguish whether the generated electric signals come from changes in the applied pressure or changes in body temperature. Also, many researchers have focused on finding low-cost and environmentally friendly materials to replace PVDF-based piezoelectric materials because of their high production costs and processing limitations. One promising candidate is polylactic acid (PLA). Compared to piezoelectric fluoropolymers, PLA is environmentally friendly, biodegradable, biocompatible, and economically advantageous. Additionally, an inexpensive PLA-based contact sensor is non-pyroelectric, shear-piezoelectric and non-aging, and may thus be effectively utilized in devices that come into close contact with the human body, unlike PVDF.
Over the past decade, the piezoelectric properties of PLA films have been studied by many researchers. Recently, PLA is receiving attention as a replacement for PVDF-based piezoelectric materials owing to the shear piezoelectric properties thereof. The shear piezoelectric properties of the PLA film are exhibited due to a chiral molecular structure having four different substituents (—O—, —COO—, —H and —CH3) and a 31 helical chain structure, formed not through a polarization process but merely through a drawing process.
FIGS. 1A to 1D illustrate the easy conversion of the 103 helix (α-crystalline form) into the 31 helix (β-crystalline form) due to the inherent helical molecular structure of the PLA drawn film. The α-crystalline form is the most thermodynamically stable at room temperature, and may be easily obtained using a melting or solution-spinning process. However, an undrawn PLA film is configured such that the molecular chain is randomly oriented and C═O dipole groups are helically oriented along the molecular chain, and thus the net dipole moment is zero, resulting in the complete loss of piezoelectric properties. Such an α-crystalline PLA film is converted into a β-crystalline conformation having a 31 helix only when subjected to uniaxial drawing at a high temperature and a high draw ratio. In this case, however, C═O dipoles are still helically oriented along the molecular chain, and thus the net dipole moment is also zero. Interestingly, when shear pressure is applied to the helical PLA molecule in the direction of a chain axis, the helix is modified through shear deformation effects, and piezoelectric current, referred to as “shear piezoelectricity,” is produced due to non-zero polar changes (FIG. 2).
The PLA monomer (lactic acid) shows a chiral structure having asymmetrical carbon. Poly(L-lactic acid) (PLLA) and poly(D-lactic acid) (PDLA) are formed through polymerization of the monomers thereof, and respectively show left-hand and right-hand 103 helices (FIGS. 3A and 3B). The stereocomplex (SC) of PLLA and PDLA was first investigated in 1987, and the structures, properties, degradations and applications thereof have been researched since. When PLLA and PDLA are formed into a stereocomplex, the chains thereof are alternately aligned side by side to form a racemic structure (FIGS. 4A and 4B). SC-PLA exhibits superior mechanical properties, thermal stability and hydrolysis resistance compared to homocrystals of PDLA and PLLA. Such characteristics enable the operation of SC-PLA-based wearable devices at high operating temperatures under harsh environmental conditions. Tsuji has studied the formation of SC from a PDLA/PLLA blend through solution casting, and Zhang et al. have studied the formation of SC by electrospinning a mixed solution of PLLA and PDLA. The homocrystals are efficiently formed in the PLLA/PDLA blend molten quenched film, whereas SC is more easily formed by the high DC voltage that is applied during electrospinning.
The electrospinning process is simple but very effective at manufacturing a flexible nanofiber web-based piezo-active device. Mandal et al. have found the piezoelectric properties in P(VDF-TrFE), as well as the relationship between dipole orientations induced by electrospinning and piezoelectric properties. FIG. 5 shows dipoles induced in the P(VDF-TrFE) jet during electrospinning and under an electric field that is applied in the electrospinning process. In the electrospinning process, high DC voltage is applied to a syringe needle and a collector. The polymer solution, discharged through the syringe needle, is stretched in an electric field and is accumulated on a metal collector. Due to the effects of stretching together with the orientation of C—F dipoles, the β-crystallinity of P(VDF-TrFE) is increased compared to a film resulting from solution casting. Accordingly, as the nanofiber web is formed, flexibility is increased, and moreover, piezoelectric properties are enhanced. However, since the stretching effect is low in the electrospinning process, the β-crystal content of the electrospun nanofiber web is low compared to the stretched film.
When a PLA nanofiber web is manufactured through electrospinning, the inherent problems of PLA films, namely brittleness and inflexibility, may be solved. Recently, Lee et al. have investigated the shear piezoelectric properties of electrospun PLLA nanofiber webs. Based on the polarized ATR spectrum data, it was verified that stretching, crystallization and dipole orientation are induced due to high DC voltage in the electrospinning process, and thus the C═O functional group of PLLA is certainly oriented. The electrospun PLLA nanofiber web may be used for capacitive nano-generators and dynamic pressure sensors through appropriate manufacturing processes, including changes in nanofiber web stacking configurations or electrode structures. However, the shear piezoelectric properties of PDLA and PLLA-PDLA racemic mixed electrospun nanofiber webs have not yet been reported.